Conventional methods of providing reflective objects typically include the use of metal or substrates coated with thin layers of metals. Forming the articles completely of metal is typically expensive and may also suffer from other disadvantages such as increased weight, etc. Metal coated articles are typically plastic substrates coated with a reflective metallic layer by vacuum, vapor or chemical deposition. These coatings suffer from a number of problems including chipping or flaking of the metallic coating, as well as corrosion of the metallic layer.
One approach to addressing the need for reflective objects has been the use of multilayer articles of polymers such as those discussed in U.S. Pat. No. 5,103,337(Schrenk et al.); U.S. Pat. No. 5,217,794 (Schrenk); U.S. Pat. No. 5,684,633 (Lutz et al.). These patents describe articles, typically films or sheets, that include multiple layers of polymers having different indices of refraction and, as a result, reflect light incident on the films. Although most of the listed patents recite that the articles are post-formable, only a few of them actually address the modifications needed to ensure that the articles retain their optical properties after forming. Among those modifications are the use of discontinuous layers (U.S. Pat. No. 5,217,794) and increasing the number of layers in the article or film (U.S. Pat. No. 5,448,404). Multilayer articles including layers of birefringent materials, their optical properties and methods of manufacturing them are disclosed in, e.g., PCT Publication Nos. WO 97/01774 and WO 95/17303. This class of articles includes alternating layers of a birefringent material and a different material in which the refractive index differential between the alternating layers is caused, at least in part, by drawing of the article, typically provided in the form of a film. That drawing causes the refractive index of the birefringent material to change, thereby causing the inter-layer refractive index differential to change. Those strain-induced refractive index differentials provide a number of desirable optical properties including the ability to reflect light incident on the films from a wide range of angles, high reflectivity over broad ranges of wavelengths, the ability to control the reflected and transmitted wavelengths, etc. For simplicity, multilayer articles including one or more layers of birefringent materials will be referred to below as “multilayer optical films.”
None of the known multilayer articles and multilayer optical films and the patents/publications describing them, however, address the problems associated with post-forming multilayer optical films. As discussed above, multilayer optical films including alternating layers of materials including at least one birefringent material rely on strain-induced refractive index differentials.
Because multilayer optical films rely on refractive index differentials developed by drawing, post-forming of multilayer optical films poses a number of problems. The additional strain caused during the post-forming processes can affect the refractive index differentials in the multilayer optical films, thereby affecting the optical properties of the multilayer optical films. For example, a multilayer optical film designed to reflect light of one polarization orientation and transmit light of the orthogonal polarization orientation may be altered during post-forming such that it reflects light with both polarization orientations. In addition, many post-forming processes involve the use of heat during forming, and that heat may alter the strain-induced crystallization that serves as the basis for the refractive index differentials in many multilayer optical films. As a result, the multilayer optical film may exhibit altered optical characteristics due to the changed refractive index differentials. Furthermore, some multilayer optical films including strain-induced birefringent layers may be stretched to levels at or near their rupture or breaking points during manufacturing. As a result, any further processing that introduces additional strain may well result in rupture of the multilayer optical films.